It has been known that carbon-carbon double bonds in polymers may be successfully hydrogenated by treating the polymer in an organic solution with hydrogen in the presence of a catalyst. Such processes can be selective in the double bonds which are hydrogenated so that, for example, the double bonds in aromatic or naphthenic groups are not hydrogenated and double or triple bonds between carbon and other atoms such as nitrogen or oxygen are not affected. This field of art contains many examples of catalysts suitable for such hydrogenations, including catalysts based on cobalt, nickel, rhodium, osmium and ruthenium. The suitability of the catalyst depends on the extent of hydrogenation required, the rate of the hydrogenation reaction and the presence or absence of other groups, such as carboxyl and nitrile groups, in the polymers.
U.S. Pat. No. 6,410,657 teaches a method for the selective hydrogenation of the unsaturated double bonds in conjugated diene units of a homopolymer or copolymer in the presence of a homogeneous organotitanium-based catalyst. It demonstrates a high degree of hydrogenation and hydrogenation reproducibility using a catalyst mixture consisting of a substituted or unsubstituted monocyclopentadienyl titanium compound and lithium hydride derived from a reaction of both alkyl lithium and hydrogen in solution.
U.S. Pat. No. 6,020,439 demonstrates a method for hydrogenating living polymers that include mainly conjugated double bond monomers and aromatic vinyl monomers. The polymer made by at least one conjugated diene compound is contacted with hydrogen in the presence of a catalyst. The catalyst is formed from a cyclopentadienyl titanium compound. A cocatalyst is provided in the form of alkoxylithium compound. This catalyst system selectively hydrogenates the unsaturated double bonds in the conjugated diene units of the living polymer in solution.
U.S. Pat. No. 5,705,571 provides a process for selective hydrogenation of a conjugated diene polymer. The process includes bringing the conjugated diene polymer in an inert organic solvent into contact with hydrogen in the presence of a hydrogenation catalyst combination including a substituted or unsubstituted bis(cyclopentadienyl) Group VIII transition metal compound and an organic lithium compound. It claims that the hydrogenation can be carried out in the presence of a small amount of the hydrogenation catalyst combination under mild conditions, and both the hydrogenation conversion and selectivity to conjugated diene units are high.
U.S. Pat. No. 5,057,581 teaches a selective hydrogenation method of the carbon-carbon double bonds of conjugated diene copolymers in homogeneous solution in an organic solvent in the presence of certain divalent ruthenium carbonyl complex catalysts containing phosphine ligands having bulky alkyl substituents.
U.S. Pat. No. 3,454,644 teaches the hydrogenation in solution of unsaturated organic compounds having from 2 to 20 carbon atoms which organic compounds contain at least one moiety selected from keto, formyl, nitrile, nonaromatic carbon double bonds and carbon-carbon triple bonds, by using as the catalyst a metal complex of ruthenium or osmium bonded to two electro negative species selected from hydrogen and halogen and complexed with at least two organic stabilizing ligands such as carbonyl or tertiary phosphine.
Guo and Rempel in Journal of Molecular Catalysis (v 63, n 3, Dec. 15, 1990, p 279-298; v 72, n 2, Mar. 1, 1992, p 193-208) describe the hydrogenation of poly cis-1,4-polybutadiene and styrene-butadiene copolymer in the presence of RhCl(PPh3)3 in solution under mild reaction conditions. Quantitative hydrogenation of carbon-carbon unsaturation can be achieved without any large scale changes in polymer chain length properties.
Mao, and Rempel in Journal of Molecular Catalysis, A: Chemical, (v 135, n 2, Oct. 14, 1998, p 121-132) teaches hydrogenation of nitrile-butadiene copolymers catalyzed in monochlorobenzene by a series of cationic rhodium complexes [Rh(diene)(L2)]+ (diene =norbornadiene (NBD) and 1,5-cyclooctadiene (COD); L2=(PPh3)2, Ph2P(CH2)nPPh2 (n=2, 3 and 4); Cy2P(CH2)2PCy2).
Charmondusit et al, in Journal of Applied Polymer Science (v 89, n 1, Jul. 5, 2003, p 142-152) describes quantitative homogeneous hydrogenation of cis-1,4-poly-(isoprene) (CPIP) in the presence of OsHCl(CO)(O2)(PCy3)2 as catalyst over the temperature range of 115-140° C. in solution.
Parent, McManus, and Rempel in Industrial & Engineering Chemistry Research, (v 37, n 11, November 1998, p 4253-4261) describes the selective hydrogenation of olefin within acrylonitrile-butadiene copolymers by homogeneous catalyst precursor, OsHCl(CO)(L)(PCy3)2 (1, L=vacant; 2, L=O2) in solution. Reversible coordination of nitrile to complex 1 not only reduces the hydrogenation rate but creates an unprecedented sensitivity of the process to pressure. Unique to this system is an apparent second-order dependence of the hydrogenation rate on [H2], which diminishes toward zero order as pressures exceed 60 bar.
Parent, McManus, and Rempel, in Industrial & Engineering Chemistry Research (v 35, n 12, December 1996, p 4417-4423) describes homogeneous catalyst precursors of the forms RhCl(PPh3)3 and RhH(PPh3)4 for the selective hydrogenation of acrylonitrile-butadiene copolymers in solution. The kinetic results suggest the behavior observed under severe conditions is consistent with that reported for pressures and temperatures near ambient. Dilute solution viscosity data are used to demonstrate the uniform selectivity of both RhCl(PPh3)3 and RhH(PPh3)4 catalyzed hydrogenations.
Pan and Rempel in Macromolecular Rapid Communications (v 25, April, 2004, p 843-847) describe an efficient hydrogenation of butadiene-styrene copolymers in solution using a ruthenium complex.
In summary, the research in this area, the hydrogenation of diene-based polymers has been very successful if the polymers were dissolved in an organic solvent.
However, many diene-based polymers/copolymers are made by emulsion polymerization processes and they are in latex form when they are discharged from polymerization reactors. Therefore it is very desirable to invent a process in which the diene-based polymer latex can be hydrogenated directly. Direct hydrogenation of polymer latex is receiving increasing attention in the recent decade. Many efforts have been made to realize such a process as discussed below.
U.S. Pat. No. 6,552,132 claims a process for the hydrogenation of a polymer composed of diene monomer units and nitrile group containing monomer units, in which the hydrogenation in the form of an aqueous dispersion in the presence of hydrazine and an oxidizing compound.
U.S. Pat. No. 6,521,694 describes a process for hydrogenating carbon-carbon double bonds of an unsaturated polymer in the form of an aqueous dispersion by adding to the unsaturated polymer (1) a reducing agent selected from the group comprising hydrazines and hydrazine-releasing compounds, (2) an oxidizing compound and (3) a catalyst, wherein the catalyst contains an element from group 13 of the Periodic Table of the Elements.
U.S. Pat. No. 5,272,202 describes a process for the selective hydrogenation of the carbon-carbon double bonds of an unsaturated, nitrile-group-containing polymer with hydrogen in the presence of a hydrogenation catalyst. An aqueous emulsion of the unsaturated, nitrile-group-containing polymer is involved. Optionally, an organic solvent capable of dissolving or swelling the polymer is caused to exist at a volume ratio of the aqueous emulsion to the organic solvent in a range of from 1:3 to 1:0. A palladium compound is used as the hydrogenation catalyst. The aqueous emulsion is brought into contact with gaseous or dissolved hydrogen while maintaining an emulsified state.
JP 02178305 describes a process for the hydrogenation of nitrile rubber by contacting the emulsions and optionally swelling the emulsions in organic solvents with hydrogen in the presence of Pd compounds. Thus, a 100 mL 10 percent nitrile rubber emulsion (containing 39.4 percent units derived from acrylonitrile) was mixed with 63.3 mg palladium benzoate in 50 mL benzene, and heated at 50° C. under 30 atm of hydrogen pressure for 6 hours to give a 90.2 percent hydrogenated emulsion.
JP 2001288212 describes a process for hydrogenated diene-based polymer latexes. Latexes of 2-chloro-1,3-butadiene (co)polymers were mixed with solutions or dispersions of catalysts in organic solvents which dissolve or swell the (co)polymers, and then contacted with hydrogen. Thus, a latex of poly(2-chloro-1,3-butadiene) rubber (Tg −42° C. and Mn 150,000) was added to a toluene solution containing RhCl(PPh3)3 and Ph3P, and hydrogenated at 100° C. and 5.0 MPa for 2 hours to give a hydrogenated polymer with Tg −54° C. and Mn 120,000.
DE 19924340 teaches a process for selective hydrogenation of nonaromatic C═C and C≡C bonds in polymers with hydrogen in the presence of Rh or Ru complexes or salts as catalysts and a nonionic P compound, such as PR3 or R2P(O)xZ(O)yPR2[R═C1-10 alkyl, C4-12 cycloalkyl, C1-10 alkoxy, aryl(oxy), F, etc.; Z=bivalent hydrocarbon residue; x, y=0, 1]. The hydrogenation is carried out in aqueous dispersions instead of in an organic medium. Aqueous dispersions containing hydrogenated polymers and their use for coatings were also claimed. Thus, an acrylic acid-butadiene-styrene copolymer latex was prepared by radical polymerization of a mixture of monomers containing also Ru(acac)3. Bu3P which was added to the latex. The system was stirred for 16 h at ambient temperature followed by hydrogenation at 150° C. and 280 bar to give a dispersion which was used to formulate a pigmented outdoor paint and a silicate-containing paint.
Guo and Rempel in Journal of Applied Polymer Science (v 65, n 4, Jul. 25, 1997, p 667-675) describes two processes for the selective hydrogenation of the C═C bonds in nitrile-butadiene rubber emulsions in the presence of a number of RuCl2(PPh3)3 complex catalysts. Both process can realize quantitative hydrogenation of the C═C, with the help of a certain amount of organic solvent to dissolve or swell the polymers.
Mudalige and Rempel in J. Molecular Catalysis (123, 15-20. 1997) describes aqueous-phase hydrogenation of polybutadiene, styrene-butadiene and nitrile-butadiene polymer emulsions catalyzed by water-soluble rhodium complexes, at 100° C. and 5.5 MPa of hydrogen pressure. The use of some organic solvent is also involved.
Zhang, et al in Hecheng Xiangjiao Gongye (2003, 26(2), 78-80) describes hydrogenation of nitrile rubber latex at ambient pressure by hydrazine hydrate/hydrogen peroxide redox system in the presence of boric acid catalyst. It was claimed that the hydrogenated nitrile latex had a degree of hydrogenation of over 90 percent when boric acid (2.64 mmol), hydrazine hydrate (95.2 mmol), reaction temperature of 60° C., and reaction time 6 hours were employed.
Wei in Guilin Gongxueyuan Xuebao (1999, 9(1), 66-69) describes a process for polyisoprene latex hydrogenation by using hydrazine hydrate and H2O2 as a redox system. The structural change of polyisoprene in hydrogenation was studied. The hydrogenated polymer from the 1,2-structure was better and the reaction rate was faster than from the 3,4- and 1,4-structures. DSC showed that the Tg of hydrogenated polyisoprene was near to that of ethylene-propylene rubber.
Singha, et al in Polym. Sci. (1994,1, 181-5) describes the use of a water-soluble analog of the Wilkinson catalyst for hydrogenation of nitrile rubber latex. The hydrogenation occurs under mild conditions (1 atm of hydrogen pressure, 75° C.) without coagulation of the latex, and up to 60 percent hydrogenation can be achieved. Gel content in latex increases with increasing catalyst concentration.
Sokolov et al in Proizvodstvo i Ispol'zovanie Elastomerov (1992, (12), 4-8) describes the effects of pH, Cu catalyst type, oxygen concentration and temperature on conversion of double bonds during hydrogenation of nitrile rubber SKN-40ASM latex with hydrazine hydrate. The maximal degree of hydrogenation was achieved at pH 8.3. CuSO4.5H2O catalyst was more active than CuCl2.xH2O. Replacement of air with oxygen in the reaction medium accelerated the process, e.g., by increasing the degree of hydrogenation after 3 h from 39.1 to 58.21 percent. Increasing the temperature from 10 to 60° C. led to a rise in side reactions, notably crosslinking.
In summary for research in this area, there are two major approaches: one approach is similar to conventional solution catalytic hydrogenation, but the polymer is hydrogenated in latex form with the presence of organic solvents instead of homogeneous organic solutions; another approach involves the use of diimide, in which a hydrogen source is generated in situ as a result of redox reactions. At present, both these approaches encounter deficiency to achieve high conversion and eliminate gel formation. The present invention is directed to a process wherein selective hydrogenation of diene-based polymer latex without any use of organic solvent, is achieved efficiently with high degree of hydrogenation and without any gel problem.